1. Field of the Invention
The present invention relates to a spread spectrum communication system capable of preventing degradation of communication performance while realizing multiplexed high-speed communication.
2. Description of the Background Art
Communication using narrow band modulation system has been conventionally used in a the field of data communication. Such a system is advantageous in that demodulation at the receiver can be carried out by a relatively small circuitry However, it has a disadvantage that it is weak against multiple path fading and narrow band color noise in a room (office, factory or the like).
By contrast, in spread spectrum communication system, spectrum of data is spread by a spread code and the data is transmitted in a wide band. Therefore, the aforementioned disadvantage can be eliminated.
In Japan, use of ISM band of 2.45 GHz has been approved and comes to be practically used. The permitted band within the ISM band of 2.45 GHz is the band width of 26 MHz, with the spreading rate of at least 10. Accordingly, when BPSK modulation is used, for example, the data rate which can be transmitted is about 1.3 MHz.
On the other hand, higher speed of communication has been desired, and for this purpose, multiplexed transmission becomes necessary. One method has been proposed for this multiplex transmission, in which data are spread by the same spreading code and the spread code are multiplex with a delay. This will be described in detail.
FIG. 123 is a schematic block diagram showing a transmitter of a conventional spread spectrum communication system. Referring to FIG. 123, the transmitter of the conventional spread spectrum communication system includes a serial/parallel converting portion 461, multipliers 463, 465, 467, delay circuits 469, 471, 473, a spread code generator (PN generator) 475, a synchronizing circuit 477, an adder 479, an amplifier 481, a local oscillator 483, a spread code generator for synchronization (PN generator for synchronization) 485, a multiplier 487 and a mixing circuit 489.
In the transmitter of the conventional spread spectrum communication system, serial baseband data S is converted to N channel parallel data by serial/parallel converting portion 461. Meanwhile, a first spread code generated by PN generator 475 is input to N type of delay circuits 469 to 473. The delay time is selected to be shorter than one cycle time of the spread code pattern, and respective delay circuits have mutually different delay times.
The PN generator 485 for synchronization generates a second spread code in synchronization with the first spread code. The second spread code is used for acquisition and tracking in synchronization with the receiving side.
The N channel parallel data output from serial/parallel converting portion 461 are subjected to spreading and modulated by the N type of first spreading code having different phases, by multipliers 463 to 467. The spread and modulated data are subjected to analog addition at adder 479 and converted to data of 1 channel.
Mixing circuit 489 mixes the output from PN generator 485 for synchronization and an output from local oscillator 483, and multiplier 487 multiplies the output from adder 479 by the output from mixing circuit 489. The multiplied data is amplified by amplifier 481 and externally transmitted through an antenna, not shown, from a duplexer , not shown, as a radio signal.
The transmitter of the conventional spread spectrum communication system described above is disclosed in Japanese Patent Laying-Open No. 4-360434. In this laid open patent, it is mentioned that delay circuits 469 to 473 have mutually different delay time. However, it is not disclosed that the delay is at least 1 chip. When the delay is 1 chip or shorter, it is possible that correlated outputs are overlapped in the receiver, degrading error rate characteristic. This will be described in a greater detail.
FIG. 124 shows correlated outputs on the receiver side, when the delay provided on the transmitting side is within 1 chip. The abscissa represents time t. Referring to FIG. 124, it can be understood that two correlated waveforms are overlapped. In this case, at a sample point SP, because of the influence of signal component (the portion represented by the arrow a) at the overlapping portion, correlated outputs are degraded, and hence error rate characteristic at the time of reception is also degraded. This is because auto-correlation characteristic of two correlated waveforms are independent when the waveforms are apart from each other by at least 1 chip, but not independent when the waveforms are not apart from each other by at least 1 chip.
In the transmitter of the conventional spread spectrum communication system, the first spread code is delayed, and the delayed first spread code is multiplied by parallel converted data to realize spreading. In this method, the point at which data changes and the start of the spreading code are not matched. This results in a disadvantage that demodulation using a correlator becomes difficult. This will be described in greater detail. FIG. 125 shows parallel converted data (N channel parallel data) and the first spread code shown in FIG. 123.
In FIG. 125, (a) represents parallel converted data at nodes a1, a2, . . . , aN of the transmitter shown in FIG. 123. The data at respective nodes a1 to aN all have the same timing.
In FIG. 125, (b) shows the first spreading code at node b of the transmitter shown in FIG. 123. It is assumed that one cycle of the spread code is T.
In FIG. 125, (c) show spread codes at nodes c1, c2, . . . cN of the transmitter shown in FIG. 123. The first spread code at nodes c1 to cN are delayed by the delay time .tau..sub.1 to .tau..sub.N, by corresponding delay circuits 469 to 473, respectively. Therefore, as shown in (a) and (c) of FIG. 125, the timings of the first delay codes are shifted from the timing of the parallel converted data. The influence when the timing of the first spreading codes are offset from the timings of the parallel converted data will be described in greater detail.
FIG. 126 is an illustration showing undesirable influence caused when the timing of the first spreading codes is offset from the timing of the parallel converted data in the transmitter.
In FIG. 126, (a) shows the parallel converted data at node a1 of FIG. 123. Here, B represents 1 bit of data.
In FIG. 126, (b) shows a spread code at node b of FIG. 123. In FIG. 126, (c) shows a spread code at node c1 of FIG. 123. As can be seen from these portions, the first spread code is delayed by .tau..sub.1 from the parallel converted data, and the timing is offset.
FIG. 126 (d) represents correlated output at a receiver not shown, when the parallel converted data and the first spread code are well timed. FIG. 126(e) shows the correlated output at the receiver, when the timing of the first spread code is offset by the delay time .tau..sub.1 from the parallel converted data.
As shown in (d) and (e) of FIG. 126, if the parallel converted data and the first spread code are off timing, the correlated output becomes smaller at the receiver. The reason for this is that the first spread code is replaced by a code of a correlator included in a receiver, not shown, as the input signal (data) is inverted midway the first spread code. This will be described in greater detail.
FIG. 127 shows specific examples of correlated outputs when the timings of the data and the first spread code are matched.
The row a represents data. The row b represents the first spread code. The first spread code includes 7 chips. More specifically, the first spread code is (1010101).
The row c represents the result of multiplication of the data by the first spread code. The row d represents correlator code possessed by the correlator of the receiver. The row e represents correlated outputs.
FIG. 128 shows specific examples of the correlated outputs at the receiver, when the timing of data is offset from the timing of the first spread code.
Referring to FIG. 128, the row a represents data. The row b represents the first spread code. The first spread code includes, similar to the first spread code shown in FIG. 127, 7 chips.
The row c represents results of multiplication of data by the first spread code. The row d represents correlator code possessed by the correlator of the receiver. The row e represents correlated output.
As can be seen from the rows a and b, the timing of the data is off from the timing of the first spread code. In FIG. 127, the correlated outputs based on the second from the left data 1 is 7. Meanwhile, in FIG. 128, the correlated output based on the second from left data 1 is 5. From the comparison between FIGS. 127 and 128, it is understood that correlated output is degraded when the data and the first spread code are off the timing. The correlated output for the third from the left data 0 is also degraded.
The reason why the correlated output based on the second from the left data is not 7 but 5 in FIG. 128 will be described. The first spread code is delayed with respect to the data. Therefore, the last information 1 (denoted by arrow f) of the second from the left spread code corresponding to the second from the left data 1 is multiplied not by the second from the left data 1 but by the next, the third from the left data 0 (denoted by the arrow g). Accordingly, as shown second from the left portion of the row c, the result of the multiplication of the spread code and the data is 1010100. By contract, in FIG. 127, as can be seen at the second from the left portion of the row c, the result of multiplication of the data by the first spread code is 1010101.
The correlator codes are the same both in FIGS. 127 and 128.
From the foregoing, the correlated output based on the second from the left data 1 of FIG. 127 is 7, while correlated output based on data 1 of FIG. 128 is 5, that is, degraded. The same applies to the correlated output based on the third from the left data 0.
From the foregoing, it becomes clear why demodulation becomes difficult when the timing of data is offset from the timing of the first spread code.
Further, in the spread spectrum communication system disclosed in Japanese Patent Laying-Open No. 5-252141, multiplexing is performed with the spread signals delayed by 1 chip or 2 chip. It is disclosed that a spread code of which side lobe of auto-correlation function attains 0 in a prescribed period is used. However, it is not described in this reference that the time is an arbitrary time period not shorter than 1 chip. Further, only a special spread code of which side lobe periodically attains 0 is used.
In this case, from the speciality of the code, the possible delay time is restricted to the point where autocorrelation of the code attains to 0, that is, either 2 chips or 1 chip. Therefore, there has been a disadvantage that freedom in designing the spread spectrum communication system is limited.
Further, when there is a delayed wave caused by multipath, for example, it may be possible that the delayed wave spread over a time period of several chips. In such a case, if the delay interval corresponds to 2 chips, delayed waves may be overlapped, resulting in degraded characteristics.
Further, as the demodulating method, the receiver of the spread spectrum communication system disclosed in Japanese Patent Laying-Open No. 4-360434 mentioned above performs active despreading by multiplying the spread code. The receiver of the spread spectrum communication system disclosed in Japanese Patent Laying-Open No. 5-252141 mentioned above uses output from a matched filter (correlator) sampled directly, as demodulation data.
In such a case, if several waves are input multiplexed with each other because of multipath, for example, what can be demodulated is only an interfered one wave of the several waves, and hence characteristics are degraded